It is known to apply heat reflective coatings to arc tube ends. See, for example, U.S. Pat. Nos. 3,226,597 and 3,325,662 where it is disclosed that, in an uncoated arc tube, metal halides can condense on the envelope wall behind the electrodes and make the lamp ineffective; such condensation is prevented by reflective coatings at the ends of the arc tube. In U.S. Pat. No. 3,889,142, there is disclosed a metal halide discharge lamp having an arc tube, the ends of which have a reflective coating thereon comprising zirconium dioxide and zirconium dibromide.
In U.S. Pat. No. 3,963,951, there is disclosed an arc tube of a high intensity arc discharge lamp which is operated horizontally and has a longitudinal stripe of heat reflecting coating along the lower surface of the arc tube to improve lamp efficiency. In U.S. Pat. No. 4,249,102, there is disclosed a halogen-metal vapor dicharge lamp in which the arc tube is at least partially frosted to provide a surface which has energy absorbent characteristics as well as light transmissive characteristics.
While such devices provide the intended results, there still exists a need to provide metal halide lamps having improved heat conservation together with improved lamp efficiency and a long life time.
Accordingly, an object of the present invention is to overcome the difficulties of the prior art. Another object of the invention is to provide a metal halide discharge lamp having reduced heat losses. Still another object of the invention is to provide an improved metal halide discharge lamp.